High-strength spring steel wire

ABSTRACT

The present invention provides a spring steel having both a high strength and a good coiling property after heat treatment, characterized by: containing, in mass, C: 0.4 to 1.2%, Si: 0.9 to 3.0%, Mn: 0.1 to 2.0%, P: 0.015% or less, S: 0.015% or less, Cr: 2.5% or less, and N: 0.001 to 0.015%, with the balance consisting of Fe and unavoidable impurities; and, in the microstructure of the steel after hot rolling, the density of globular cementite carbides 0.2 to 3 μm in circle-equivalent diameter being 0.5 piece/μm 2  or less and the density of globular cementite carbides over 3 μm in circle-equivalent diameter being 0.005 piece/μm 2  or less.

TECHNICAL FIELD

The present invention relates to a steel and a steel wire having a highstrength and a high toughness after heat treatment and suitable forsprings for cars and general machinery.

BACKGROUND ART

As the trend to weight reduction and higher performance of cars hasgrown, the strength of springs used for cars has been enhanced and, as aresult, a high-strength steel having a tensile strength exceeding 1,600MPa after heat treatment is being used for springs. Recently, a steelhaving a tensile strength exceeding 1,900 MPa has been used for thisapplication.

There are two methods in manufacturing coiled springs using a steel: thehot coiling method wherein a steel is heated to the austenitizingtemperature range, coiled and then quenched and tempered; and the coldcoiling method wherein a high-strength steel wire quenched and temperedbeforehand is coiled in a cold state. In either case, the fundamentalstrength of a spring is determined by quenching and tempering and, forthis reason, the design of a chemical composition in consideration ofthe properties after quenching and tempering is an important factor inmanufacturing a spring steel.

Specifically, according to Japanese Unexamined Patent Publication No.S57-32353, hardenability is improved by adding V, Nb, Mo and otherelements, and setting resistance is improved by forming fine carbidesprecipitated during tempering and thereby restricting the movement ofdislocations.

Among the aforementioned two methods in manufacturing coiled steelsprings, namely, the hot coiling method wherein a steel is heated to theaustenitizing temperature range, coiled and then quenched and temperedand the cold coiling method wherein a high-strength steel wire quenchedand tempered beforehand is coiled in a cold state, in the case of thecold coiling method, an oil quenching and tempering treatment or a highfrequency heat treatment, wherein rapid heating and rapid cooling can beapplied during the production of steel wires, is employed, and it ispossible to make the size of prior austenite grains in a spring steelmaterial fine, and thus springs excellent in fracture resistance can bemanufactured. In addition, as an installation such as a heating furnaceor the like can be simplified in a spring manufacturing line, the coldcoiling method has the advantage of reducing the equipment cost incurredby a spring manufacturer or the like. For this reason, the springmanufacturing by the cold coiling method is common.

However, as the strength of a steel wire for cold-coiled springs hasincreased, the steel wire tends to break during the cold coiling and itoften becomes impossible to form the steel wire into the shape of aspring. Therefore, it has not always been possible to enjoy a highstrength and a good workability at the same time and, for this reason,an industrially disadvantageous manufacturing method has had to beemployed for the coiling work. In case of manufacturing a valve spring,though, usually, a steel wire is subjected to an on-line quenching andtempering treatment, namely an oil quenching and tempering treatment,and then coiled in a cold state, a method is employed wherein a steelwire is heated to a temperature at which it is easily deformed in orderto prevent the steel wires from breaking during the coiling and,thereafter, subjecting it to a tempering treatment in order to obtain ahigh strength as disclosed, for instance, in Japanese Unexamined PatentPublication No. H05-179348 wherein a steel wire is heated to 900 to1,050° C., coiled and then tempered at 425 to 550° C. However, theheating before coiling and the tempering after coiling sometimes cause adimensional fluctuation of a product spring depending on the conditionof heat treatment and/or a remarkable deterioration of treatmentefficiency, and, for this reason, a spring manufactured by this methodis inferior to that manufactured by the cold coiling method in terms ofcost and dimensional accuracy.

In the production processes of a steel, the steel is repeatedlysubjected to heating and cooling several times, for example in theprocesses of converter refining, casting, billet rolling and wire rodrolling. During these processes, carbide forming elements such as Cr, V,Nb and Mo contained in a steel harden the steel and, at the same time,they are likely to be retained in the steel in the form of coarsecarbides. In particular, when a high strength exceeding 1,900 MPa interms of tensile strength is expected, the addition amount of thesealloying elements tends to increase and the amount of retained carbidesincreases accordingly. In view of the situation, Japanese UnexaminedPatent Publication No. H11-6033 and some others pay attention to thecarbides of Cr, V, Nb, Mo, etc. (hereinafter referred to as “alloycarbides”), and propose to regulate the grain size of the alloycarbides. However, the behavior that actually determines the strength ofa steel is not the behavior of the fine carbides of these elements, butthe behavior of carbides mainly composed of cementite, namely ironcarbides, (hereinafter referred to as “cementite carbides”), and,therefore, the control of the very cementite is important for a springsteel.

With regard to the grain size of alloy carbides, proposals that payattention to the average grain size of the carbides of Nb and V areadvanced, for example, in Japanese Unexamined Patent Publication No.H10-251804. However, in this prior art, there is a description ofapprehending that an abnormal structure is generated by cooling waterduring rolling (paragraph 0015), and the prior art substantiallyrecommends dry rolling.

However, dry rolling, which is distinctly different from normal rolling,is an unstable operation in industrial practices and the prior artsuggests that, even when the average grain size is controlled by dryrolling, if unevenness occurs in the peripheral matrix structure,rolling troubles are caused, and that, as a consequence, the controlalone of the average grain size of the alloy carbides such as thecarbides of V and Nb is not sufficient, industrially.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a spring steel and asteel wire, for springs, producable industrially and capable of securingthe strength and coiling property suitable for a spring after quenchingand tempering.

The present inventors have developed a spring steel and a steel wire forsprings having both a high strength and a good coiling property afterquenching and tempering by making the size of carbides, and cementite inparticular, in a steel fine, which size has not been an object ofspecial attention in conventional spring steel technologies. That is,the gist of the present invention is the following spring steel andsteel wire for springs.

-   -   (1) A high-strength spring steel and a steel wire for springs        characterized by: containing, in mass,        -   C: 0.4 to 1.2%,        -   Si: 0.9 to 3.0%,        -   Mn: 0.1 to 2.0%,        -   P: 0.015% or less,        -   S: 0.015% or less,        -   Cr: 2.5% or less, and        -   N: 0.001 to 0.015%,            with the balance consisting of Fe and unavoidable            impurities; and, in the microstructure of the steel after            hot rolling, the density of globular cementite carbides 0.2            to 3 μm in circle-equivalent diameter being 0.5 piece/μm² or            less and the density of globular cementite carbides over 3            μm in circle-equivalent diameter being 0.005 piece/μm² or            less.    -   (2) A high-strength spring steel and a steel wire for springs        according to the item (1), characterized by further containing,        in mass, one or more of;        -   W: 0.05 to 1.0%,        -   Co: 0.05 to 5.0%,        -   Ti: 0.005 to 0.1%,        -   Mo: 0.05 to 1.0%,        -   V: 0.05 to 0.7%,        -   Nb: 0.01 to 0.05%,        -   B: 0.0005 to 0.006%,        -   Ni: 0.05 to 5.0%,        -   Cu: 0.05 to 0.5%, and        -   Mg: 0.0002 to 0.01%.    -   (3) A high-strength spring steel characterized by: containing,        in mass,        -   C: 0.4 to 0.8%,        -   Si: 0.9 to 3.0%,        -   Mn: 0.1 to 2.0%,        -   P: 0.015% or less,        -   S: 0.015% or less,        -   Cr: 1.5% or less, and        -   N: 0.001 to 0.007%,            with the balance consisting of Fe and unavoidable            impurities; and, in the microstructure of the steel after            hot rolling, the density of globular cementite carbides 0.2            to 3 μm in circle-equivalent diameter being 0.5 piece/μm² or            less and the density of globular cementite carbides over 3            μm in circle-equivalent diameter being 0.005 piece/μm² or            less.    -   (4) A high-strength spring steel according to the item (3),        characterized by further containing, in mass, one or more of;        -   W: 0.05 to 1.0%,        -   Co: 0.05 to 5.0%,        -   Ti: 0.005 to 0.1%,        -   Mo: 0.05 to 1.0%,        -   V: 0.05 to 0.7%,        -   Nb: 0.01 to 0.05%,        -   B: 0.0005 to 0.006%,        -   Ni: 0.05 to 5.0%,        -   Cu: 0.05 to 0.5%, and        -   Mg: 0.0002 to 0.01%.    -   (5) A high-strength spring steel characterized by: containing,        in mass,        -   C: 0.8 to 1.0%,        -   Si: 0.9 to 3.0%,        -   Mn: 0.1 to 2.0%,        -   P: 0.015% or less,        -   S: 0.015% or less,        -   Cr: 0.5% or less,        -   N: 0.001 to 0.007%, and            further one or both of W: 0.05 to 1.0% and Co: 0.05 to 5.0%,            with the balance consisting of Fe and unavoidable            impurities; and, in the microstructure of the steel after            hot rolling, the density of globular cementite carbides 0.2            to 3 μm in circle-equivalent diameter being 0.5 piece/μm² or            less and the density of globular cementite carbides over 3            μm in circle-equivalent diameter being 0.005 piece/μm² or            less.    -   (6) A high-strength spring steel according to the item (5),        characterized by further containing, in mass, one or more of;        -   Ti: 0.005 to 0.1%,        -   Mo: 0.05 to 1.0%,        -   V: 0.05 to 0.7%,        -   Nb: 0.01 to 0.05%,        -   B: 0.0005 to 0.006%,        -   Ni: 0.05 to 5.0%,        -   Cu: 0.05 to 0.5%, and        -   Mg: 0.0002 to 0.01%.    -   (7) A high-strength spring steel characterized by containing, in        mass,        -   C: 0.8 to 1.2%,        -   Si: 0.9 to 3.0%,        -   Mn: 0.1 to 2.0%,        -   P: 0.015% or less,        -   S: 0.015% or less,        -   Cr: 0.5 to 1.5%,        -   N: 0.001 to 0.015%, and        -   W: 0.05 to 1.0%,            with the balance consisting of Fe and unavoidable            impurities.    -   (8) A high-strength spring steel according to the item (7),        characterized by further containing, in mass, one or more of;        -   Ti: 0.005 to 0.1%,        -   Mo: 0.05 to 1.0%,        -   V: 0.05 to 0.7%,        -   Nb: 0.01 to 0.05%,        -   B: 0.0005 to 0.006%,        -   Ni: 0.05 to 5.0%,        -   Cu: 0.05 to 0.5%, and        -   Mg: 0.0002 to 0.01%.    -   (9) A high-strength spring steel according to the item (7) or        (8), characterized by, in the microstructure of the steel after        hot rolling, the density of globular cementite carbides 0.2 to 3        μm in circle-equivalent diameter being 0.5 piece/μm² or less and        the density of globular cementite carbides over 3 μm in        circle-equivalent diameter being 0.005 piece/μm² or less.    -   (10) A high-strength heat-treated steel wire for springs        characterized by: containing, in mass,        -   C: 0.4 to 1.0%,        -   Si: 0.9 to 3.0%,        -   Mn: 0.1 to 2.0%,        -   P: 0.015% or less,        -   S: 0.015% or less,        -   Cr: 2.5% or less, and        -   N: 0.001 to 0.007%,            with the balance consisting of Fe and unavoidable            impurities; the tensile strength (TS) being 1,900 MPa or            more; the percentage of the area occupied by the globular            cementite carbides 0.2 μm or more in circle-equivalent            diameter on a microscopic observation plane being 7% or            less; the density of the globular cementite carbides 0.2 μm            or more in circle-equivalent diameter being 1 piece/μm² or            less; the density of the globular cementite carbides over 3            μm in circle-equivalent diameter being 0.001 piece/μm² or            less; the grain size number of prior austenite being 10 or            higher; the maximum grain size of carbides being 15 μm or            less; and the grain size of oxides being 15 μm or less.    -   (11) A high-strength heat-treated steel wire for springs        according to the item (10), characterized by further containing,        in mass, one or more of;        -   W: 0.05 to 1.0%,        -   Co: 0.05 to 5.0%,        -   Ti: 0.005 to 0.1%,        -   Mo: 0.05 to 1.0%,        -   V: 0.05 to 0.7%,        -   Nb: 0.01 to 0.05%,        -   B: 0.0005 to 0.006%,        -   Ni: 0.05 to 5.0%,        -   Cu: 0.05 to 0.5%, and        -   Mg: 0.0002 to 0.01%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph showing the structure of a quenched and temperedspring steel.

FIG. 2 shows the results of analyzing globular alloy carbides in springsteels according to the present invention by EDX using an SEM: FIG. 2(a) shows the result in the case where a steel contains C of 0.4 to 0.8%;FIG. 2( b) the case where a steel contains C of 0.8 to 1.0%; FIG. 2( c)the case where a steel contains C of 0.4 to 1.0%; and FIG. 2( d) thecase where a steel contains C of 0.75 to 1.2%.

FIG. 3 shows the results of analyzing globular cementite carbides inspring steels according to the present invention by EDX using an SEM:FIG. 3( a) shows the result in the case where a steel contains C of 0.4to 0.8%; FIG. 3( b) the case where a steel contains C of 0.8 to 1.0%;FIG. 3( c) the case where a steel contains C of 0.4 to 1.0%; and FIG. 3(d) the case where a steel contains C of 0.75 to 1.2%.

FIGS. 4( a) and 4(b) are illustrations showing the notch bending testmethod.

FIGS. 5( a) and 5(b) are illustrations showing the delayed fracture testmethod.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides a spring steel and a heat-treated steelwire for springs wherein a high strength is secured by specifyingchemical components appropriately and a good coiling performance duringthe production of springs is secured by controlling the shape ofcarbides in the steel by means of heat treatment.

In the first place, the reasons for specifying the chemical componentsof a steel are explained.

C is an element which exerts a great influence on the fundamentalstrength of a steel material and, as such, the range of its content isset at 0.4 to 1.2% for obtaining a sufficient strength. If the contentof C is below 0.4%, a sufficient strength is not realized and, to makeup for the insufficient strength, the contents of the other alloyingelements must be increased. If it exceeds 1.2%, coarse cementite grainsprecipitate in a great amount after normal rolling and, as a result,toughness is remarkably deteriorated. The deterioration of toughnessdeteriorates the coiling property at the same time and, in addition,leads to the problems of requiring a higher heat treatment temperaturein the industrial production of a spring steel and making a highfrequency heat treatment difficult.

Further, in consideration of the relation with the other alloyingelements and the methods of heat treatment such as oil quenching andtempering and high frequency heat treatment, it is desirable to controlthe content of C within the range at 0.4 to 1.2%.

Si is an element indispensable for securing the strength, hardness andsettling resistance of a spring and, when its content is below 0.9%, arequired strength and settling resistance cannot be obtained and, forthis reason, the lower limit of its content is set at 0.9%. Besides theabove, Si has the effects of spheroidizing the precipitates of carbidesat grain boundaries and making the precipitates fine and thus has theeffect of reducing the percentage of the area occupied by intergranularprecipitates in the grain boundaries. However, its addition in a greatamount not only hardens a steel material but also embrittles it. Forthis reason, in order to avoid the embrittlement after quenching andtempering, the upper limit of Si content is set at 3.0%.

Mn enhances hardenability and hardens the matrix of a steel. It alsomakes S harmless by fixing S in a steel in the form of MnS. With regardto the behavior of carbides, which is one of the points of specialimportance in the present invention, Mn is also an element capable ofsecuring strength without forming carbides. For this reason, it isnecessary to add Mn by 0.1% or more for fixing S in the form of MnS and,for securing strength, it is desirable to add Mn by 0.5% or more. Theupper limit of its content is set at 2.0% for preventing theembrittlement caused by Mn.

Cr is an element effective for improving hardenability and tempersoftening resistance. When the fatigue strength of a spring is enhancedby surface hardening in nitriding treatment, as the Cr contentincreases, the hardened layer becomes thicker in a short nitridingtreatment time and the maximum hardness is also likely to be higher. Forthis reason, it is desirable to add Cr when nitriding is employed in themanufacturing process. When the addition amount of Cr is large, however,the manufacturing cost increases and, besides, cementite which appearsafter quenching and tempering is made coarse and, as a consequence, aproduced steel wire tends to be brittle and prone to break duringcoiling. As Cr dissolves in cementite precipitating after rolling, inparticular, the cementite is stabilized and is likely to be insolubleduring heating for quenching. This fact exerts a significant influenceon an oil-tempered steel wire, a high frequency heat-treated materialand so on. Therefore, the upper limit of Cr content is set at 2.5%, theamount beyond which cementite is rendered hardly soluble during theheating for quenching in a spring manufacturing process and the heattreatment during the production of a spring or a steel wire for springsbecomes very difficult.

It is preferable to control the addition amount of Cr to 1.5% or less,but the amount should be controlled in consideration of the relationswith the other addition elements and a method of heat treatment such asan oil quenching and tempering treatment or a high frequency heattreatment. That is, when C amount is 0.8% or more, it is desirable tocontrol the Cr content to 0.5% or less and/or add simultaneously theelements such as W and Co, which are capable of suppressing theformation of globular cementite carbides.

N combines with V and Nb to form nitrides and, at the same time,facilitates the formation of carbonitrides. As the carbonitrides serveas the pinning grains to suppress the growth of austenite grains duringquenching, N is effective also for making the size of austenite grainsfine. For these purposes, N is added by 0.001% or more. An excessiveamount of N, on the other hand, causes the coarsening of nitrides,carbonitrides generated by making the nitrides work as nuclei, andcarbides. For this reason, the upper limit of the content is set at0.007%.

P makes steel hard but, on the other hand, it forms segregation andembrittles a material. In particular, P segregated at austenite grainboundaries lowers impact values and causes delayed cracking and the likeby the penetration of hydrogen, and therefore the smaller the content,the better. For the above reasons, the upper limit of the content is setat 0.015%, the amount beyond which the embrittlement becomesconspicuous.

S, like P, also embrittles a steel when contained in a steel. Itsadverse effect is reduced to the utmost by adding Mn but, as MnS alsotakes the form of inclusions, a fracture property is deteriorated. In ahigh-strength steel in particular, only a small amount of MnS may causefracture and, for this reason, it is desirable to make its content assmall as possible. Therefore, its upper limit is set at 0.015%, theamount beyond which the adverse effect becomes significant.

W improves hardenability and, at the same time, enhances strength byforming carbides in steel. W is particularly important because it iscapable of suppressing the coarsening of cementite and other alloycarbides. When the addition amount is below 0.05%, no tangible effect isobtained but, when it exceeds 1.0%, coarse carbides form and, adversely,ductility and other mechanical properties may be deteriorated. For thisreason, the content is limited to the range at 0.05 to 1.0%.

Co lowers hardenability but it has an effect of securing strength at ahigh temperature. Besides, it is effective for suppressing the formationof carbides and, in particular, it suppresses the formation of coarsecarbides, which constitutes one of the main issues of the presentinvention. When the content is below 0.05%, the effect is too small but,when it exceeds 5.0%, the effect is saturated. For this reason, Cocontent is limited to the range at 0.05 to 5.0%.

Both W and Co have the effect of making the cementite carbides fine,although their mechanisms, in a steel, are different. Therefore, when asteel contains a great amount of C as in the case of the presentinvention, they are effective for making cementite grains fine andeasily soluble in the steel. For this reason, when a high-C steel isproduced, it is desirable to add W and/or Co.

Ti, Mo, V and Nb precipitate in a steel in the forms of nitrides,carbides and carbonitrides. Thus, when one or more of these elements areadded, the precipitates form and temper softening resistance isobtained, which fact makes it possible to secure a high strength withoutsoftening even when a steel is subjected to a heat treatment such asstress relief annealing in a high temperature tempering process ornitriding. As this suppresses the deterioration of the internal hardnessof a spring after nitriding and/or facilitates hot setting and stressrelief annealing, the fatigue property of the spring finally produced isimproved. However, if the addition amount of Ti, Mo, V and Nb is toohigh, the size of their precipitates becomes too large, the precipitatescombine with carbon in a steel and coarse carbides are formed. Thisdecreases the amount of C to contribute to the strengthening of a steelwire and, as a consequence, the strength proportionate to the additionamount of C is not obtained. What is more, as the coarse carbides act asthe sites of stress concentration, a steel wire easily deforms or breaksduring coiling work.

Ti often takes the form of precipitates already in molten steel becausethe precipitation temperature of Ti nitrides is high. In addition, Ti isalso used for fixing N in a steel because the bonding strength with N isstrong. When B is added to a steel, it is necessary to add Ti in theamount enough for fixing N for preventing B from forming BN. Thus, it isdesirable to fix N using Ti. The lower limit of the addition amount ofTi is set at 0.005% because this is the minimum required addition amountfor making austenite grains fine, and the upper limit is set at 0.1%because this is the maximum amount in which the size of the precipitatesdoes not adversely affect the fracture property.

Mo, when added at 0.05 to 1.0%, improves hardenability, gives tempersoftening resistance to a steel, and makes it possible to raise thetempering temperature when steel strength is controlled. This isadvantageous for lowering the percentage of the area occupied byintergranular carbides in the grain boundaries. In other words, Mo iseffective in spheroidizing intergranular carbides, which precipitate inthe shape of films, through tempering at a high temperature and thusdecreasing the percentage of the area occupied by intergranular carbidesin the grain boundaries. Besides, Mo forms Mo carbides in a steel,separately from cementite. In particular, because the precipitationtemperature of Mo is lower than that of V and other elements, Mo has aneffect of suppressing the coarsening of carbides. When the additionamount of Mo is below 0.05%, said effects do not show up, but theeffects are saturated when Mo is added in excess of 1.0%.

V is useful for suppressing the coarsening of austenite grains by theformation of nitrides, carbides and carbonitrides and, in addition, forhardening a steel wire at a tempering temperature as well as hardeningthe surface layer during nitriding. Little effect is obtained when theaddition amount of V is below 0.05% and, when it exceeds 0.7%, coarseinsoluble inclusions are formed and toughness is deteriorated.

Likewise, Nb is useful for suppressing the coarsening of austenitegrains by the formation of nitrides, carbides and carbonitrides and, inaddition, for hardening a steel wire at a tempering temperature as wellas hardening the surface layer during nitriding. As Nb forms finecarbides at a temperature higher than the temperature at which V, Mo andso on form fine carbides, the effect of fining the size of austenitegrains during the manufacturing of a heat-treated steel wire is largeeven with a small addition amount and, thus, Nb is a very usefulelement. When the content is below 0.01%, little tangible effect isobtained and, when the content exceeds 0.05%, coarse insolubleinclusions are formed and toughness is deteriorated. For this reason,the range of the content is set at 0.01 to 0.05%.

B is known as an element to enhance hardenability. It is also effectivefor cleaning austenite grain boundaries. This means that the elementssuch as P and S, which precipitate at grain boundaries and lowertoughness, are rendered harmless by the addition of B and, thus, thefracture property is improved. If B combines with N to form BN, however,the above effect is lost. The lower limit of the addition amount of B isset at 0.0005%, the amount at which the effect becomes obvious, and theupper limit at 0.006%, the amount beyond which the effect is saturated.

Ni improves hardenability and enhances strength stably through a heattreatment. It also improves the coiling property by enhancing theductility of a steel matrix. In addition, it increases the corrosionresistance of a spring and, thus, it is effective for a spring used in acorrosive environment. When its addition amount is below 0.05%, theeffect of improving strength and ductility is not obtained but, when itscontent exceeds 5.0%, the effect is saturated and such an addition of Niis disadvantageous in terms of manufacturing costs and so on.

As for Cu, its addition prevents decarburization. As the existence of adecarburized layer decreases the fatigue life of a spring after it ismanufactured, efforts are exerted to make the decarburized layer assmall as possible and, when the decarburized layer is thick, the surfacelayer is removed by peeling work. Cu also has an effect of improvingcorrosion resistance, like Ni. Therefore, by suppressing the formationof a decarburized layer, the fatigue life of a spring is improved andthe peeling process can be eliminated. The effects of Cu in suppressingdecarburization and improving corrosion resistance manifest themselveswhen Cu is added by 0.05% or more. When Cu content exceeds 0.5%, even ifNi is added as explained later, a steel is embrittled and rollingdefects are likely to occur. For these reasons, the lower limit of Cucontent is set at 0.05%, and the upper limit at 0.5%. While it is littlelikely that the mechanical properties of a steel at room temperature areadversely affected by the addition of Cu, when Cu is added in excess of0.3%, hot ductility is deteriorated and, as a result, cracks may developat the surface of a billet during rolling. As a countermeasure, it isdesirable to control the addition amount of Ni, which prevents cracksduring rolling, in relation to the addition amount of Cu so as tosatisfy the expression [Cu %]<[Ni %].

Mg is an oxide forming element and, as such, forms oxides in moltensteel. The temperature range in which the oxides of Mg are formed ishigher than that of MnS and, therefore, when MnS is formed, the Mgoxides already exist in molten steel. Therefore, the Mg oxides can beused as precipitation nuclei of MnS and the distribution of MnS can becontrolled using this mechanism. That is to say, as Mg oxides aredispersed in molten steel in finer grains than the oxides of Si and Alcommonly seen in a conventional steel, MnS formed by making Mg oxideswork as precipitation nuclei is dispersed in the steel in fine grains.As a consequence, the distribution of MnS varies depending on whether Mgis contained or not even though S content is identical and, when Mg isadded, the grain size of MnS becomes finer. A sufficient effect isobtained with only a small amount of Mg: MnS grains are made fine when0.0002% or more of Mg is added. However, industrially, 0.01% is theupper limit of its addition because Mg of 0.01% or more is hard toretain in molten steel. Thus, the range of the addition amount of Mg isset at 0.0002 to 0.01%. It is desirable to add Mg whenever possible,because Mg is effective in improving corrosion resistance and delayedfracture and preventing rolling cracks and so on by virtue of the MnSdistribution and the like. A preferable range of the addition amount ofMg is at 0.0005 to 0.01%.

Issues related to the production of a spring steel according to thepresent invention envisaging a higher strength than conventional steelsare explained hereafter. Springs are strengthened through quenching andtempering, but steels of conventional chemical compositions areembrittled during the process and are not suitable for actual use. Forthis reason, the temperature of tempering has to be lowered, orotherwise, when springs are manufactured by the cold coiling method,steel wires break during the coiling work after the quenching andtempering. The amount of C is increased to some extent and/or alloyingelements such as Cr and V are added as common countermeasures againstthe above problems. However, when the addition amount of the alloyingelements is increased, segregation occurs and, as the melting point islocally lowered at the portions where they are densely concentrated,cracks are likely to develop there. This is considered to be one of thecauses of the rolling defects.

Next, carbides, which constitute one of the points of special note inthe present invention, are explained. When the performance of a springis examined, the form of carbides in a steel is an important factor.Here, the carbides in a steel mean cementite seen in a steel after heattreatment, carbides formed by the solution of alloying elements incementite (hereinafter collectively referred to as “cementite”) andalloy carbides. The carbides can be observed by polishing to a mirrorfinish and etching a sectional plane of a steel or a steel wire.

FIG. 1 shows a micrograph of a typical example of a structure formed byquenching and tempering. Two kinds of carbides, i.e., carbidesprecipitated in a pearlite shape or a tabular shape and globularcarbides, are seen in the steel in FIG. 1. Spring steels are cast,rolled into billets, cooled once to the room temperature and then rolledinto wire rods of the sizes required by purchasers. The spring steelsare further quenched and tempered and, in this process, while thecementite in the tabular or pearlite shape easily dissolves into steel,the carbides which are spheroidized and stabilized do not easilydissolve during the subsequent quenching and tempering process. For thisreason, the strength proportionate to the addition amount of C may notbe obtained and/or the ductility for coiling work may be lowered. Theglobular carbides also cause the rolling defects during the rolling of awire rod.

The retained insoluble carbides do not in the least contribute to thestrength and toughness obtainable through quenching and tempering, andthey not only waste the added C by fixing C in a steel but also act asthe sites of stress concentration, and therefore they cause thedeterioration of the mechanical properties of a steel wire. The globularcarbides are products formed by growing, globularly, as they failed todissolve in a steel during reheating (such as heating during the rollingof wire rods, spring manufacturing and so on) after cooling. Therefore,it is preferable to make the amount of globular carbides as small aspossible even immediately after the rolling of a wire rod. The globularcarbides further grow and coarsen particularly during the heat treatmentafter rolling such as oil quenching and tempering treatment. From thisviewpoint, even the carbides 3 μm or less in circle-equivalent diameter,generally considered to be harmless, are highly likely to causeproblems. The present inventors discovered that cementite composedmainly of Fe and C, which had not thitherto attracted attention, was notan exception in this respect. These coarse and insoluble carbides notonly adversely affect the manufacturing of a spring but also causedefects during rolling.

The cementite carbides also include the substances wherein alloyingelements such as Cr, Mo, etc. are dissolved in cementite; generallyspeaking, the cementite containing these elements in a solute state isstable and hardly dissolved in steel. The detection of the cementite iscarried out by analyzing the carbides which appear after etching, and,in this case, carbides mainly composed of Fe and C are detected anddissolved alloying elements are also detected, sometimes. Hereunder,this type of carbides mainly composed of Fe and C is referred to ascementite carbides and, in particular, when it takes a globular shape,as globular cementite carbides.

FIGS. 2( a) to 2(d) show examples of the carbides analyzed using an EDXattached to an SEM. Similar analysis results can be obtained by thereplica method using a transmission electron microscope. Priorinventions have paid attention only to the carbides of the alloyingelements such as V and Nb added for obtaining a high strength, and thistype of carbides is characterized by having a very low Fe peak in thecarbides as exemplified in FIG. 2( a). In contrast, the presentinvention has paid attention not only to the precipitation of thecarbides of alloying elements noticed in the prior inventions but alsoto that of the globular cementite carbides 3 μm or less incircle-equivalent diameter composed of Fe₃C and Fe₃C wherein a smallamount of alloying elements is dissolved, as shown in FIGS. 3( a) to3(d). When it is attempted to achieve both a higher strength and abetter workability than a conventional steel wire as in the case of thepresent invention, if the amount of the globular cementite carbides 3 μmor less in circle-equivalent diameter is large, workability issignificantly deteriorated.

When a steel is coiled after being quenched and tempered, the globularcementite carbides affect the coiling property of the steel, namely thebend ability of the steel until fracture occurs. Attention has hithertobeen paid to the view that, when not only C but also alloying elementssuch as Cr and V are added in a large amount for obtaining a highstrength, coarse globular carbides are generated abundantly and causethe deterioration of coiling property and the generation of rollingdefects. However, it is the so-called globular cementite carbidescomposed of Fe₃C and Fe₃C wherein a small amount of alloying elements isdissolved, as shown in FIGS. 3( a) to 3(d), that affect the rollingdefects and the coiling property, and, when the globular cementitecarbides exist in a great amount and/or they grow into coarse grains,they promote the occurrence of cracks during rolling and, at the sametime, deteriorate the mechanical properties of a heat-treated steel wireand their coiling property in particular.

The globular carbides can be observed by polishing in mirror finish andetching a sectional plane of a sample using picral or the like. For theobservation of the object carbides, namely the globular cementitecarbides 0.2 to 3 μm in circle-equivalent diameter, and the detailedevaluation of their size and so on, it is necessary to observe themunder a high magnification of 3,000 or more using a scanning electronmicroscope. It was believed that fine carbides in a steel wereindispensable for securing the strength and temper softening resistanceof a steel, but the present inventors discovered that the grain sizeeffective for the purposes was 0.1 μm or less in terms ofcircle-equivalent diameter and that, when their circle-equivalentdiameter exceeded 1 μm, they would not contribute to strengthening asteel and making the austenite grains fine any more, but would merelydeteriorate the deformation property.

In addition, when the grain size (circle-equivalent diameter) ofglobular cementite carbides is 3 μm or less, not only their size butalso their number constitutes a significant factor in the presentinvention. Therefore, in consideration of the aspects of their size andnumber, the present inventors found out that, even if thecircle-equivalent diameter was as small as from 0.2 to 3 μm, when theirdensity on a microscopic observation plane was so large as to exceed 0.5piece/μm², the coiling property deteriorated significantly.

What is more, when the size of the carbides exceeds 3 μm, the influenceof the size becomes larger and, as a consequence, when their density ona microscopic observation plane exceeds 0.005 piece/μm², the coilingproperty deteriorates remarkably.

If such carbides remain immediately after hot rolling, they do noteasily dissolve in a steel during various heat treatments in thesubsequent processes from wire drawing to spring manufacturing and, forthis reason, it is desirable that they do not remain insolubleimmediately after the wire rod rolling, too. Based on the above, thepresent invention stipulates that, in the microstructure of a steelafter rolling, the density of the globular cementite carbides 0.2 to 3μm in circle-equivalent diameter is 0.5 piece/μm² or less and thedensity of the globular cementite carbides over 3 μm incircle-equivalent diameter is 0.005 piece/μm² or less.

What is more, when the percentage of the area occupied by the globularcementite carbides on a microscopic observation plane exceeds 7%,regardless of their size, the coiling property is so heavilydeteriorated that the coiling work is rendered impossible. For thisreason, the present invention stipulates that the percentage of the areaoccupied by them on a microscopic observation plane is 7% or less.

On the other hand, in addition to carbides, the size of prior austenitegrains has a significant influence on the fundamental properties of asteel wire; the smaller the prior austenite grain size is, the betterthe fatigue property and coiling property become. However, no matter howsmall a prior austenite grain size is, if carbides are contained in asteel beyond the upper limit specified in the present invention, theabove effect is impaired. Generally speaking, in order to make a prioraustenite grain size small, it is effective to lower the heatingtemperature but, adversely, this increases carbides. Therefore, it isimportant to produce a steel wire wherein a carbide amount and a prioraustenite grain size are well balanced with each other. Here, if a prioraustenite grain size number is below 10, even when the carbides arewithin the range specified above, a sufficient fatigue property is notobtained. For this reason, the present invention stipulates that theprior austenite grain size number must be 10 or higher.

In addition, when both the maximum grain size of all the carbides,including the carbides of alloying elements and so on, and the maximumgrain size of oxides exceed 15 μm, the fatigue property is deteriorated.For this reason, they are limited in the present invention by settingtheir respective upper limits at 15 μm.

It has to be noted that, while wire rods are produced through theprocesses of continuous casting, billet rolling and wire rod rolling, orthe processes of continuous casting and wire rod rolling, a steel iscooled to below A₁ transformation point between each of the productionprocesses and, for this reason, carbides precipitate as early asimmediately after the continuous casting. Therefore, for decreasing theglobular cementite carbides retained after wire rod rolling, it isnecessary to secure a high temperature and a long time sufficient fordissolving the coarse carbides in a steel in the heating of the billetrolling and that of the wire rod rolling.

EXAMPLE Example 1

Table 1 shows examples according to the present invention andcomparative examples.

Steels were melted and refined in a 250-t converter and produced intobillets by continuous casting. In the other examples, steels were meltedand refined in a 2-t vacuum melting furnace and then produced intobillets by rolling. The steels of the invented examples were held at ahigh temperature of 1,200° C. or more for a prescribed time. Then, ineither case, the billets were rolled into bars 8 mm in diameter and thendrawn into wires 4 mm in diameter. In the comparative examples, on theother hand, the billets were rolled under normal rolling conditions and,then, subjected to wire drawing.

In the tables, heat treatment methods, OT, IQT and F indicate oil tempertreatment, high frequency quenching and tempering, and quenching andtempering using an off-line batch furnace (a radiation furnace),respectively.

Since the present invention features excellent properties in rollingdefect property and material properties after the post-rolling quenchingand tempering, quite different from those obtained through conventionaltechnologies, the material properties were evaluated after rolling andalso after quenching and tempering. The existence or otherwise ofrolling defects was observed visually immediately after rolling.

After being drawn to a diameter of 4 mm, the steel wires were quenchedand tempered through so-called oil quenching and tempering treatment,wherein they were quenched by being heated in a radiation furnace and,immediately thereafter, cooled in an oil bath and then tempered by beingheated in a molten Pb bath.

In the oil quenching and tempering treatment, the drawn wires were madeto pass through a heating furnace continuously, and the residence timein the heating furnace was controlled so that the wires were well heatedto the center portion. If the heating is insufficient, hardening becomesinsufficient and a sufficient strength is not obtained. In this example,the heating temperature was set at 950° C., the heating time at 150 sec.and the quenching temperature (oil bath temperature) at 50° C. Then, thestrength of the wires was controlled through the tempering at atempering temperature of 400 to 550° C. and a tempering time of 1 min.The heating temperature for the quenching and tempering and the tensilestrength in the normal atmosphere obtained through the treatment were asshown in Table 1. The tensile strength was controlled to 2,100 MPa orso.

The globular carbides in a steel including cementite, which isconsidered to be important in the present invention, are also listed inthe table. The size and number of the carbides were evaluated bypolishing a longitudinal sectional plane of a hot-rolled wire rod or anas-heat-treated steel wire in mirror finish and, then, lightly etchingthe plane with picric acid so that the carbides appear on the sectionalplane. Since it is difficult to measure the size of the carbides at themagnification level of an optical microscope, micrographs of 10observation fields were taken at random at the portion of ½ R of ahot-rolled wire rod and a steel wire under a magnification of 5,000using a scanning electron microscope. Then, in the micrographs, globularcementite carbides were identified from among globular carbides using anX-ray microanalyzer attached to the scanning electron microscope, andtheir size and number were measured using an image processor. Thecircle-equivalent diameter of each of the globular carbides and theirdensity were calculated from the data thus obtained. The total measuredarea was 3,088.8 μm².

The tensile property of a sample was tested in compliance with JapaneseIndustrial Standard (JIS) Z 2241 using No. 9 test pieces according toJIS Z 2201, and tensile strength was calculated from the breaking forceat the test.

The ductility of a sample was evaluated by a notch bending test in thefollowing manner, an outline of which is shown in FIG. 4. As seen inFIG. 4( a), a groove 2 (notch) 30 μm in maximum depth running in rightangles to the longitudinal direction of a steel wire was cut in a testpiece using a punch having an apex radius of 50 μm, the test piece washeld at both the ends so that the maximum tensile load was imposed onthe notched portion, three-point bending deformation force was appliedto the test piece by imposing a load 3 at the center, the bendingdeformation force was imposed until the test piece broke at the notchedportion, and the bending angle at the time of the breakage shown in FIG.4( b) was measured. The larger the measured angle (θ), the better thecoiling property. Empirically, if the notch bending angle of a steelwire 4 mm in diameter is 250 or less, coiling work is difficult.

In the invented examples, the size of globular carbides after rollingwas small, no rolling defects were seen, and a high strength and a goodnotch bending property were realized after the quenching and tempering.It is seen from the table that comparative examples, in contrast, wereinferior in the notch bending property and the coiling property.Further, rolling defects were found in them, witnessing a difficulty inthe rolling operation.

Table 2 shows the chemical compositions, the existence or otherwise ofrolling defects, the density of the carbides 0.2 to 3 μm incircle-equivalent diameter, the density of the carbides over 3 μm incircle-equivalent diameter, the tensile strength and the coilingproperty (the reduction of area at the tensile test) of the invented andcomparative examples rolled initially into wire rods 15 mm in diameter,drawn to a diameter of 12 mm and then oil-tempered.

After being drawn to the diameter of 12 mm, the samples underwentannealing at 400° C. for 20 min. simulating the oil quenching andtempering and the stress relief annealing in spring manufacturing sothat their tensile strength was controlled to 1,950 to 2,000 MPa.

The evaluation results of the steel wires 12 mm in diameter in Table 2show that, in comparative examples having a carbide density outside therange of the present invention, the reduction of area, which is anindicator of the coiling property, is small and, when the density of thecarbides over 3 μm in circle-equivalent diameter is 0.005 piece/μm² ormore immediately after rolling, the carbides are retained after the heattreatment and deteriorate the reduction of area. This is suspected tohave caused an inferior coiling property.

From Tables 1 and 2, it is understood that, in the steels outside theranges of the present invention, basically, rolling defects are likelyto persist. The globular cementite carbides are suspected to be largelyresponsible for this; globular cementite carbides exceeding the rangespecified in the present invention were detected in the observation ofthe sound portions of these steels after rolling. This indicates that,even when austenite is formed through two or more heatings, if carbidesremain in the precursor structure, cracks and other defects are likelyto occur easily during subsequent processes such as wire rod rolling,coiling after oil quenching and tempering treatment and hot coiling.

[Tables 1 and 2]

TABLE 1 Chemical composition Density Roll- Tensile Notch ing (afterrolling) de- Density (after OT) strength bending Example No. C Si Mn P SCr W Co Ti V Mo Nb Ni Cu B Mg N 0.2-3 <3 fects 0.2-3 <3 MPa angle°Invented 1 0.70 1.46 0.77 0.015 0.004 0.20 — 0.0009 0.0048 0.0630 0.0016∘ 0.179 0.0008 2107 38 example Invented 2 0.67 1.91 1.11 0.009 0.0130.17 — 0.0006 0.0049 0.0045 0.0005 ∘ 0.022 0.0018 2132 37 exampleInvented 3 0.64 1.99 0.32 0.006 0.003 0.18 0.17 0.068 0.0009 0.00410.0427 0.0009 ∘ 0.140 0.0006 2106 37 example Invented 4 0.73 1.93 0.510.004 0.004 0.30 0.07 0.19 0.0005 0.0035 0.0695 0.0010 ∘ 0.177 0.00072060 38 example Invented 5 0.63 2.15 0.31 0.012 0.004 0.16 0.19 0.00100.0048 0.0702 0.0010 ∘ 0.199 0.0008 2136 34 example Invented 6 0.70 1.920.41 0.012 0.010 0.20 0.29 0.44 0.0005 0.0036 0.0272 0.0006 ∘ 0.1080.0006 2159 39 example Invented 7 0.63 1.89 1.12 0.013 0.011 0.27 0.200.039 0.0010 0.0048 0.0247 0.0015 ∘ 0.072 0.0013 2089 36 exampleInvented 8 0.61 1.86 1.20 0.014 0.009 0.16 0.13 0.44 0.0005 0.00430.0174 0.0013 ∘ 0.056 0.0010 2055 33 example Invented 9 0.66 1.65 0.730.010 0.007 0.29 0.22 0.23 0.19 0.0009 0.0036 0.0126 0.0007 ∘ 0.0620.0013 2128 38 example Invented 10 0.73 1.83 0.98 0.010 0.010 0.29 0.150.0011 0.0038 0.0115 0.0014 ∘ 0.052 0.0008 2090 32 example Invented 110.68 1.64 0.80 0.010 0.010 0.16 0.12 0.033 0.0028 0.0011 0.0036 0.02590.0017 ∘ 0.106 0.0011 2155 39 example Invented 12 0.60 2.04 1.16 0.0130.004 0.16 — — 0.0043 0.0611 0.0008 ∘ 0.183 0.0017 2095 38 exampleInvented 13 0.67 2.14 0.95 0.007 0.011 0.28 — 0.09 0.22 0.0007 0.00350.0618 0.0007 ∘ 0.156 0.0012 2094 35 example Invented 14 0.62 1.59 0.370.011 0.008 0.18 — 0.10 0.29 0.0010 0.0045 0.0300 0.0012 ∘ 0.137 0.00112159 39 example Invented 15 0.63 2.08 0.44 0.005 0.004 0.29 — 0.14 0.300.05 0.0007 0.0027 0.0395 0.0015 ∘ 0.152 0.0016 2096 38 example Invented16 0.62 2.06 0.90 0.014 0.010 0.19 — 0.14 0.09 0.13 0.049 0.0008 0.00270.0390 0.0014 ∘ 0.108 0.0006 2143 38 example Invented 17 0.69 1.82 0.730.008 0.011 0.24 — 0.22 0.13 0.07 0.044 0.0009 0.0028 0.0356 0.0010 ∘0.117 0.0007 2088 33 example Invented 18 0.71 2.19 0.47 0.004 0.005 0.17— 0.25 0.15 0.22 0.046 0.47 0.0007 0.0034 0.0166 0.0017 ∘ 0.052 0.00102065 33 example Invented 19 0.73 2.25 0.97 0.014 0.007 0.24 0.18 0.070.26 0.014 0.0027 — 0.0034 0.0311 0.0006 ∘ 0.149 0.0019 2132 35 exampleInvented 20 0.64 1.92 0.11 0.005 0.013 0.29 0.22 0.29 0.09 0.048 0.0023— 0.0040 0.0386 0.0010 ∘ 0.122 0.0016 2148 33 example Invented 21 0.692.09 1.16 0.010 0.008 0.25 0.29 0.21 0.16 0.026 0.0016 — 0.0032 0.04250.0006 ∘ 0.129 0.0008 2155 37 example Compar- 22 0.84 2.14 0.93 0.0120.012 1.19 — 0.35 0.10 0.130 0.0044 0.4016 0.0092 x ative exampleCompar- 23 0.95 1.87 0.67 0.015 0.015 1.09 — 0.11 0.09 0.044 — 0.00430.5304 0.0122 x ative example Compar- 24 0.64 2.20 0.43 0.012 0.014 2.15— 0.09 0.30 — 0.0034 0.2999 0.0134 ∘ 1.447 0.0005 2063 20 ative exampleCompar- 25 0.69 1.34 0.24 0.003 0.008 2.51 — 0.13 0.21 0.041 — 0.00340.7540 0.0093 ∘ 1.060 0.0015 2111 22 ative example Compar- 26 0.89 2.010.78 0.005 0.011 0.54 — 0.56 0.45 0.48 — 0.0050 0.2488 0.0090 x ativeexample

TABLE 2 Chemical composition Density Roll- Density Tensile Reduc- ingtion (after rolling) de- (after OT) strength of area Example No. C Si MnP S Cr W Co Ti V Mo Nb Ni Cu B Mg N 0.2-3 <3 fects 0.2-3 <3 MPa %Invented 27 0.52 2.32 0.97 0.013 0.007 0.88 0.07 0.0020 0.0033 0.03880.001 ∘ 0.187 0.0011 1974 52.4 example Invented 28 0.49 1.70 0.34 0.0100.006 0.82 0.12 0.15 0.021 0.0009 0.0037 0.0730 0.002 ∘ 0.190 0.00151975 48.4 example Invented 29 0.47 2.03 0.59 0.013 0.011 1.07 0.28 0.130.17 0.157 0.25 0.11 0.0012 0.0041 0.0594 0.001 ∘ 0.192 0.0010 1960 54.0example Invented 30 0.48 1.67 0.24 0.003 0.009 0.84 0.10 0.25 0.05 0.100.065 0.25 0.0024 0.0007 0.0048 0.0210 0.001 ∘ 0.079 0.0007 1971 48.1example Invented 31 0.46 1.72 1.09 0.013 0.011 0.80 — 0.09 0.16 0.2250.028375 0.0011 0.0049 0.0034 0.001 ∘ 0.014 0.0012 1967 50.6 exampleInvented 32 0.52 1.83 0.36 0.009 0.011 0.91 — 0.11 0.256 0.028139 0.00220.0005 0.0043 0.0639 0.001 ∘ 0.184 0.0009 1967 51.0 example Invented 330.55 1.83 0.14 0.015 0.003 0.90 — 0.12 0.266 0.021847 0.0019 0.00110.0028 0.0468 0.001 ∘ 0.177 0.0006 1960 50.3 example Compar- 34 0.571.46 1.02 0.005 0.010 0.89 — 0.12 0.57 0.510 0.101 — 0.0049 0.566  0.0132 x ative example Compar- 35 0.56 1.55 0.33 0.010 0.007 2.21 —0.08 0.56 0.430 0.07 — 0.0035 0.895   0.0150 ∘ 1.11  0.0010 1958 22.7ative example

Example 2

Table 3 shows examples according to the present invention andcomparative examples.

The same procedures as in Example 1 were applied to the steels of theexamples according to the present invention and comparative examplesfrom the melting and refining of the steel to the production of thesteel wires.

The rolling defects and the properties after the post-rolling quenchingand tempering and the defects immediately after the rolling wereevaluated also in the same manner as in Example 1.

After being drawn to a diameter of 4 mm, the steel wires were quenchedand tempered through a so-called oil quenching and tempering treatment,wherein they were quenched by being heated in a radiation furnace and,immediately thereafter, cooled in an oil bath and then tempered by beingheated in a molten Pb bath.

In the oil quenching and tempering treatment, the drawn wires were madeto pass through a heating furnace continuously, and the residence timein the heating furnace was controlled so that the wires were well heatedto the center portion. If the heating is insufficient, hardening isinsufficient and it becomes impossible to attain a sufficient strength.In this example, the heating temperature was set at 950° C., the heatingtime at 150 sec. and the quenching temperature (oil bath temperature) at50° C. Then, the strength of the wires was controlled through thetempering at a tempering temperature of 400 to 550° C. and a temperingtime of 1 min. The heating temperature for the quenching and temperingand the tensile strength in the normal atmosphere obtained through thetreatment were as shown in Table 3. The tensile strength was controlledto 2,150 to 2,250 MPa or so.

In the invented examples, the number and size of globular carbides afterrolling were small, no rolling defects were seen, and a high strengthand a good notch bending property were realized after the quenching andtempering. It is seen from the table that comparative examples, incontrast, were inferior in the notch bending property and the coilingproperty. Further, rolling defects were found in them, witnessingdifficulty in rolling operation.

Both W and Co are considered to have the effect of suppressing theformation of coarse cementite grains, although their behaviors in steelare different: Co suppresses the formation of carbides, whereas Wsuppresses the growth of cementite, and thus the coarsening issuppressed.

[Table 3]

TABLE 3 Chemical composition Density Density Tensile Notch (afterrolling) Rolling (after OT) strength bending Example No. C Si Mn P S CrW Co Ti V Mo Nb Ni Cu B Mg N 0.2-3 <3 defects 0.2-3 <3 MPa angle°Invented 36 0.82 1.77 0.85 0.007 0.008 0.34 0.13 — 0.0009 0.0028 0.0190.0020 ∘ 0.052 0.0007 2170 33 example Invented 37 0.84 1.86 0.84 0.0090.005 0.44 0.28 — 0.0008 0.0028 0.015 0.0008 ∘ 0.050 0.0013 2245 32example Invented 38 0.82 1.71 0.80 0.006 0.013 0.42 0.06 — 0.023 0.00070.0044 0.005 0.0008 ∘ 0.022 0.0009 2249 30 example Invented 39 0.82 1.790.76 0.009 0.009 0.44 0.13 — 0.29 0.0005 0.0037 0.005 0.0016 ∘ 0.0160.019  2206 28 example Invented 40 0.81 1.89 1.08 0.011 0.008 0.34 0.18— 0.15 0.0005 0.0030 0.004 0.0006 ∘ 0.013 0.012  2206 28 exampleInvented 41 0.81 1.76 0.91 0.008 0.006 0.48 0.18 0.14 0.19 0.0010 0.00280.035 0.0005 ∘ 0.108 0.005  2206 30 example Invented 42 0.81 1.84 0.910.013 0.005 0.41 0.22 0.28 0.041 0.0004 0.0046 0.007 0.0019 ∘ 0.0250.0013 2153 33 example Invented 43 0.84 1.83 1.06 0.011 0.005 0.35 0.240.20 0.46 0.0004 0.0036 0.031 0.0009 ∘ 0.090 0.0005 2186 29 exampleInvented 44 0.81 1.86 1.06 0.015 0.005 0.35 0.26 0.25 0.33 0.31 0.00050.0047 0.008 0.0008 ∘ 0.030 0.0015 2151 29 example Invented 45 0.85 1.720.93 0.009 0.013 0.39 0.17 0.29 — 0.0027 0.044 0.0013 ∘ 0.113 0.00132218 31 example Invented 46 0.84 1.71 0.95 0.011 0.004 0.36 0.14 0.150.031 0.0021 — 0.0044 0.067 0.0008 ∘ 0.177 0.0006 2248 31 exampleInvented 47 0.81 2.29 0.21 0.007 0.003 0.27 0.20 0.28 0.031 — 0.00320.219 0.0012 ∘ 0.609 0.0018 2270 30 example Invented 48 0.91 2.15 1.160.009 0.012 0.47 0.28 — 0.0008 0.0028 0.189 0.0009 ∘ 0.626 0.0017 221232 example Invented 49 0.83 1.77 0.60 0.006 0.005 0.24 0.13 — 0.00070.0048 0.145 0.0020 ∘ 0.418 0.0018 2226 28 example Invented 50 0.80 2.041.13 0.011 0.011 0.41 0.15 — — 0.0032 0.036 0.0019 ∘ 0.177 0.0009 220530 example Invented 51 0.80 1.41 0.30 0.007 0.012 0.10 — 0.21 0.20 0.310.0010 0.0045 0.266 0.0018 ∘ 0.726 0.0008 2239 30 example Invented 520.86 2.28 0.53 0.013 0.007 0.18 — 0.15 0.48 0.0007 0.0036 0.041 0.0007 ∘0.142 0.0018 2214 28 example Invented 53 1.09 2.30 1.13 0.009 0.013 0.48— 0.26 0.41 0.32 0.0012 0.0030 0.013 0.0009 ∘ 0.059 0.0019 2172 29example Invented 54 0.89 2.30 0.19 0.008 0.013 — 0.27 0.27 0.34 0.230.0006 0.0046 0.110 0.0010 ∘  0.0393 0.0019 2230 33 example Invented 550.99 1.82 0.23 0.008 0.007 — 0.16 0.24 0.48 0.23 0.0006 0.0030 0.0080.0012 ∘ 0.034 0.0014 2192 30 example Invented 56 0.98 2.27 0.78 0.0040.008 — 0.24 0.29 0.30 0.40 0.0007 0.0041 0.102 0.0016 ∘ 0.420 0.00082190 31 example Compar- 57 0.91 1.72 0.97 0.014 0.005 1.56 — — 0.45 0.280.0046 0.680 0.0091 x ative example Compar- 58 0.98 2.02 1.02 0.0050.013 0.80 — — 0.47 0.30 0.0046 0.398 0.0104 ∘ 1.340 0.0115 2263 19ative example Compar- 59 0.87 2.12 0.20 0.012 0.007 0.45 — — 0.28 0.480.16 0.21 0.0042 0.460 0.0084 ∘ 1.800 0.0093 2208 21 ative exampleCompar- 60 0.74 2.15 0.52 0.006 0.008 0.96 0.16 — 0.28 0.31 0.0030 0.0060.0120 ∘ 0.011 0.0132 2163 20 ative example Compar- 61 0.76 1.98 0.400.003 0.009 1.56 0.18 0.16 0.26 0.16 0.0049 0.156 0.0139 x ative example

Example 3

Table 4 shows the chemical compositions, the heat treatment method, thepercentage of the area occupied by globular cementite carbides, thedensity of the globular cementite carbides 0.2 to 3 μm incircle-equivalent diameter, the density of the globular cementitecarbides over 3 μm in circle-equivalent diameter, the maximum diameterof carbides, the maximum diameter of oxides, the prior austenite grainsize number, the tensile strength, the coiling property (notch bendingangle) and the average fatigue strength (rotary bending) of the inventedand comparative examples drawn to a diameter of 4 mm.

In examples 36 and 53, steels were melted and refined in a 250-tconverter and produced into billets by continuous casting. In the otherexamples, steels were melted and refined in a 2-t vacuum melting furnaceand then produced into billets by rolling. The steels of the inventedexamples were held at a high temperature of 1,200° C. or more for aprescribed time. Then, in either case, the billets were rolled into bars8 mm in diameter and then drawn into wires 4 mm in diameter. In thecomparative examples, on the other hand, the billets were rolled undernormal rolling conditions and, then, subjected to wire drawing.

In view of the fact that a carbide amount and steel strength varydepending on a chemical composition, the invented examples wereheat-treated according to their respective chemical compositions so thattheir tensile strength became 2,100 to 2,200 MPa or so and the otherproperty figures fell within their respective ranges according to thepresent invention. On the other hand, comparative examples wereheat-treated so that only their tensile strength fell within the rangeaccording to the present invention.

In the treatment in a batch furnace, test pieces 1 m in length werestraightened and quenched by heating in a heating furnace and makingthem pass through an oil tank kept at 60° C. The heating time was set at30 min. and the temperature history was adjusted so as to correspond tothat of the hot-coiled springs manufactured by the hot coiling method.After the quenching, the test pieces were charged again into a heatingfurnace for tempering in order to control the tensile strength in normalatmosphere. The heating temperature for the quenching and tempering andthe tensile strength in normal atmosphere obtained through the quenchingand tempering were as shown in Table 3.

In the oil quenching and tempering treatment, the drawn wires were madeto pass through a heating furnace continuously, and the residence timein the heating furnace was controlled so that the wires were well heatedto the center portion. In this example, the heating temperature was setat 950° C., the heating time at 150 sec. and the quenching temperature(oil bath temperature) at 50° C. Then, the strength of the wires wascontrolled through the tempering at a tempering temperature of 400 to550° C. and a tempering time of 1 min. The heating temperature for thequenching and tempering and the tensile strength in the normalatmosphere obtained through the treatment were as shown in Table 3.

In the high frequency heat treatment, the test pieces were heated bymaking them pass through an induction coil and then water-cooledimmediately after passing through the coil. The heating temperature was990° C., the heating time 15 sec., and the test pieces were quenchedthrough the water-cooling (room temperature). Thereafter, they weretempered at a tempering temperature of 430 to 600° C. by making thempass through the coil again. The tensile strength in the normalatmosphere thus obtained was as shown in Table 1.

The steel wires thus prepared were subjected to the evaluation ofcarbides, the tensile test and the notch bending test without furthertreatment. As for the evaluation of the fatigue property, test pieceswere prepared by applying a heat treatment at 400° C. for 20 min.simulating the stress relief annealing in spring manufacturing, a shotpeening treatment (for 20 min. with cut wires 0.6 mm in diameter) andthen a low temperature stress relief annealing at 180° C. for 20 min.

The evaluation of the size and number of carbides and the notch bendingtest were done in the same manner as in Example 1.

The fatigue test was done using a Nakamura's rotary bending tester,wherein the maximum load stress under which 10 test pieces withstood 10⁷cycles or more of bending with a probability of 50% or more was used asthe average fatigue strength.

Table 5 shows the chemical compositions, the heat treatment method, thepercentage of the area occupied by globular cementite carbides, thedensity of the globular cementite carbides 0.2 to 3 μm incircle-equivalent diameter, the density of the globular cementitecarbides over 3 μm in circle-equivalent diameter, the prior austenitegrain size number, the tensile strength, the coiling property (thereduction of area in the tensile test), the fatigue strength, and thedelayed fracture strength of the invented and comparative examples drawnto a diameter of 12 mm.

In the examples, the steels were melted and refined in a 2-t vacuummelting furnace and then produced into billets by rolling. Thereafter,in either case, the billets were rolled into the bars 14 mm in diameterand then drawn into the wire rods 12 mm in diameter.

In this case, since the test pieces were thicker than the test pieces 4mm in diameter, the reduction of area in the tensile test was used asthe indicator of the coiling property.

The fatigue strength was evaluated through the fatigue test using anOno's rotary bending tester, and the fatigue limit was used as thefatigue strength.

The test method of evaluating delayed fracture strength is shown in FIG.5. FIG. 5( a) shows the shape of a test piece 4. Using the delayedfracture tester shown in FIG. 5( b), load 5 is imposed on the test piece4 having a circumferential notch in a vessel while hydrogen is charged,and the time until the breakage occurs under the condition is measured.The test piece 4 is held in a solution 7 (H₂SO₄, pH 3.0) kept at 30° C.by a band heater, and is tested using a constant current power source 8(at a current density of 1.0 mA/cm²) using the test piece as the cathodeand a platinum electrode as the anode 9. When the load is changed insuch a test, the maximum load W which the test piece withstands for aloading time of 200 h. can be measured. The nominal stress (or W/S)calculated by dividing the maximum load W by the sectional area S at thebottom of the circumferential notch was used as the value of the delayedfracture strength.

In the case that the steel wires were drawn to a diameter of 12 mm, thetest pieces for the fatigue test and the delayed fracture test underwentonly annealing at 400° C. for 20 min. simulating the stress reliefannealing. The shot peening treatment and the subsequent stress reliefannealing, which were applied to the samples 4 mm in diameter, wereomitted.

As seen in Table 4, in the case that the steel wires were drawn to adiameter of 4 mm, in the comparative examples having the percentage ofthe area occupied by the globular cementite carbides and the density ofthe globular cementite carbides falling outside their respective rangesspecified in the present invention, the bending angle at the notchbending test, which is an indicator of the coiling property, is toosmall for successful coiling work even though the chemical compositionis within the range of the present invention. On the other hand, evenwhen the specifications regarding carbides are satisfied, if strength isnot sufficient, fatigue strength becomes insufficient and the materialcannot be used for high-strength springs.

As seen in the evaluation results of the steel wires 12 mm in diametershown in Table 5, in comparative examples having the percentage of thearea occupied by the globular cementite carbides and the density of theglobular cementite carbides falling outside their respective rangesspecified in the present invention, the reduction of area, which isanother indicator of the coiling property, is small even when thechemical composition is within the range of the present invention. Inaddition, when strength is lowered in an attempt to improve the above,fatigue strength deteriorates. The grain size of austenite has aninfluence on fatigue and delayed fracture properties; in the case thataustenite grain size was large, even though the specifications regardingcarbides were satisfied, the fatigue and delayed fracture propertieswere insufficient.

It has to be noted, however, that the austenite grain size can bedecreased by measures such as lowering the heating temperature ofquenching and making the heating time short but, as these measures leavemany carbides in an insoluble state, adversely, it becomes difficult tosatisfy the specifications of the present invention. For this reason,the introduction of a technology to enable a short time, hightemperature heating for dissolving carbides such as oil quenching andtempering and high frequency heat treatment is important and, as seen inTables 1 and 2, it is difficult to realize both a high strength and agood coiling property at the same time by means of a fast but imperfecttreatment in a batch furnace. This also means that the production ofhigh-strength springs is difficult using the low temperature short timequenching.

[Tables 4 and 5]

TABLE 4 Prior Max. auste- Not- Heat Area car- Max. nite Ten- tch Ro-treat- per- bide oxide grain sile bend- tary ment cent- grain grain sizestre- ing bend- Exam- Chemical composition meth- age Density size sizenum- ngth an- ing ple No. C Si Mn P S Cr Ti V Nb Mo W Ni Cu Co B Mg N od% 0.2-3 >3 μm μm ber MPa gle° MPa In- 62 0.75 2.19 0.80 0.008 0.010 0.970.0006 0.0029 OT 3.2 0.26 <0.0001 12.4 12.0 11 2098 33 850 ven- ted ex-am- ple In- 63 0.83 2.42 0.25 0.004 0.006 0.06 0.0009 0.0052 OT 0.2 0.03<0.0001 12.0 12.7 11 2118 28 852 ven- ted ex- am- ple In- 64 0.83 1.780.24 0.004 0.008 0.51 0.01  0.0007 0.0025 OT 2.7 0.06 <0.0001 12.5 11.713 2165 34 861 ven- ted ex- am- ple In- 65 0.89 1.61 0.26 0.004 0.0040.26 0.2 0.0008 0.0049 OT 0.3 0.09 <0.0001 10.3 10.5 12 2118 31 860 ven-ted ex- am- ple In- 66 0.84 1.49 1.03 0.010 0.011 0.48 0.02 0.00070.0025 OT 0.7 0.15 <0.0001 12.3 10.4 13 2118 32 865 ven- ted ex- am- pleIn- 67 0.75 1.82 1.02 0.006 0.006 0.87 0.3 0.0008 0.0050 OT 0.5 0.30<0.0001 12.6 11.2 11 2156 32 882 ven- ted ex- am- ple In- 68 0.69 1.851.07 0.005 0.011 1.27 0.1 0.0010 0.0034 OT 3.6 0.220 <0.0001 11.0 12.612 2201 28 850 ven- ted ex- am- ple In- 69 0.73 1.21 1.10 0.010 0.0070.56 0.5 0.0010 0.0022 OT 0.0 0.20 <0.0001 12.1 11.7 13 2166 31 859 ven-ted ex- am- ple In- 70 0.71 1.64 1.18 0.007 0.008 0.99 0.2 0.2 0.00080.0031 OT 0.8 0.40 <0.0001 10.9 10.3 11 2117 33 877 ven- ted ex- am- pleIn- 71 0.71 2.10 0.73 0.009 0.004 0.55 0.2 0.0007 0.0057 OT 1.7 0.04<0.0001 12.9 12.6 10 2159 33 860 ven- ted ex- am- ple In- 72 0.75 2.520.95 0.008 0.009 0.62 0.017 0.002 0.0009 0.0030 OT 1.9 0.17 <0.0001 11.811.0 10 2114 33 860 ven- ted ex- am- ple In- 73 0.86 1.77 0.70 0.0100.009 0.51 0.5 0.0007 0.0055 OT 1.9 0.16 <0.0001 10.8 10.0 13 2178 30856 ven- ted ex- am- ple In- 74 0.75 1.21 0.35 0.008 0.010 0.68 0.00120.0056 IQT 0.5 0.08 <0.0001 10.8 11.6 13 2109 29 863 ven- ted ex- am-ple In- 75 0.75 2.20 0.95 0.011 0.010 0.47 0.0009 0.0034 IQT 0.3 0.11<0.0001 11.5 12.4 13 2120 32 853 ven- ted ex- am- ple In- 76 0.65 2.030.69 0.003 0.003 0.18 — 0.0045 IQT 0.7 0.007 <0.0001 12.8 10.3 12 215234 852 ven- ted ex- am- ple In- 77 0.83 1.79 0.37 0.008 0.011 0.39 0.10.1 0.1 0.0005 0.0027 OT 6.3 0.25 <0.0001 11.0 12.7 12 2244 32 881 ven-ted ex- am- ple In- 78 0.75 2.32 0.26 0.008 0.011 0.07 0.04 0.00100.0047 OT 0.4 0.04 <0.0001 10.1 12.0 10 2108 29 852 ven- ted ex- am- pleIn- 79 0.78 1.36 0.88 0.006 0.006 0.49 0.1 0.1 0.0008 0.0023 IQT 0.10.15 <0.0001 10.8 10.1 12 2154 30 863 ven- ted ex- am- ple In- 80 0.791.58 0.73 0.012 0.007 1.33 0.3 0.1 0.2 0.1 0.0035 OT 6.5 0.34 <0.000111.5 12.8 11 2252 33 866 ven- ted ex- am- ple In- 81 0.69 2.53 0.680.006 0.006 0.71 0.2 0.01 0.2 0.1 0.2 0.0022 OT 0.2 0.14 <0.0001 10.111.9 11 2189 30 872 ven- ted ex- am- ple In- 82 0.70 2.60 0.65 0.0050.011 0.02 0.15 0.1 0.15 0.0058 OT 0.1 0.01 <0.0001 10.4 10.1 10 2165 31850 ven- ted ex- am- ple Com- 83 0.78 1.45 0.82 0.008 0.009 1.09 0.350.06 0.1 0.0031 OT 9.2 0.43 <0.0001 13.0 11.2 12 2183 22 861 par- ativeex- am- ple Com- 84 0.82 1.68 0.38 0.003 0.003 0.82 0.11 0.09 0.0034 IQT9.9 1.34 <0.0001 11.7 11.9 13 2173 19 878 par- ative ex- am- ple Com- 850.68 1.21 0.66 0.007 0.010 1.68 0.35 0.02 0.21 0.0041 OT 8.0 1.50<0.0001 12.4 11.4 11 2245 20 872 par- ative ex- am- ple Com- 86 0.771.88 0.93 0.003 0.006 1.48 0.1 0.22 0.0042 OT 4.4 0.24 0.02 12.2 10.3 102178 33 792 par- ative ex- am- ple Com- 87 0.76 2.57 0.96 0.010 0.0120.54 0.15 0.0023 F 0.4 0.03 <0.0001 11.3 10.4  9 2119 18 857 par- ativeex- am- ple Com- 88 0.78 2.58 0.40 0.007 0.004 1.68 0.45 0.12 0.160.0023 F 1.6 0.26 <0.0001 20.0 12.7 12 2162.1 21 855 par- ative ex- am-ple Com- 89 0.69 1.35 0.54 0.005 0.011 0.75 0.03 0.19 0.0032 F 0.1 0.01<0.0001 11.5 25.5 12 2167.9 22 798 par- ative ex- am- ple

TABLE 5 Chemical composition Example No. C Si Mn P S Cr Ti V Nb Mo W NiCu C B Mg N Invented 90 0.50 2.21 0.26 0.005 0.003 0.98 0.0009 0.0043example Invented 91 0.55 2.61 0.89 0.007 0.007 1.32 0.05 0.1 0.1 0.00210.0007 0.0043 example Invented 92 0.57 1.52 0.41 0.008 0.006 1.24 0.10.0011 0.0045 example Invented 93 0.62 1.73 1.19 0.004 0.010 1.34 0.20.0006 0.0028 example Invented 94 0.64 1.84 0.13 0.004 0.008 0.35 0.20.2 0.0012 0.0032 example Invented 95 0.53 1.85 0.28 0.004 0.005 0.930.5 0.0008 0.0032 example Invented 96 0.52 1.83 1.16 0.008 0.009 1.370.3 0.0007 0.0028 example Comparative 97 0.63 1.49 0.71 0.011 0.004 1.390.52 0.21 0.036 example Comparative 98 0.50 2.68 0.65 0.009 0.009 0.610.05 0.4 0.06 0.2 0.034 example Comparative 99 0.51 2.13 0.91 0.0070.004 0.88 0.043 example Comparative 100  0.58 1.77 0.50 0.012 0.0041.35 0.035 example Comparative 101  0.59 1.24 0.46 0.003 0.004 0.200.023 example Comparative 102  0.53 1.90 0.38 0.007 0.011 0.77 0.024example Comparative 103  0.51 2.65 0.31 0.005 0.006 0.51 0.0038 exampleFatigue Max. Max. Prior strength Delayed Heat Area carbide oxideaustenite Tensile at Ono's fracture treatment percentage Density graingrain grain size strength Reduction of test strength Example No. method% 0.2–3 >3 size μm size μm number MPa area % MPa MPa Invented 90 OT 1.40.28 <0.0001 12.0 10.1 12 1951 44 945 1025 example Invented 91 OT 0.60.60 <0.0001 10.6 11.8 12 1999 50 932 1030 example Invented 92 OT 4.90.50 <0.0001 11.4 10.8 13 1958 40 968 1071 example Invented 93 OT 4.10.53 <0.0001 12.0 12.5 10 1949 41 946  915 example Invented 94 OT 0.60.19 <0.0001 10.6 13.0 13 2004 49 940 1036 example Invented 95 OT 3.00.03 <0.0001 10.8 11.8 10 1952 44 880  919 example Invented 96 OT 1.40.54 <0.0001 10.9 11.1 11 1962 41 968  959 example Comparative 97 OT 8.50.27 <0.0001 11.9 10.7 12 2019 32 967 980 example Comparative 98 OT 1.21.37 <0.0001 11.2 12.4 11 1986 33 942 966 example Comparative 99 OT 1.00.08 <0.0001 12.3 11.6 12 1971 50 890 872 example Comparative 100  F 0.70.56 <0.0001 12.3 13.0  8 1985 32 881 859 example Comparative 101  OT0.8 0.07 <0.0001 11.6 12.2 10 1812 48 851 873 example Comparative 102 OT 0.9 0.26 <0.0001 22.2 12.2 11 1979 31 882 873 example Comparative103  OT 0.3 0.23 <0.0001 12.4 31.3 11 1980 48 843 921 example

Example 4

Tables 6 and 7 show examples according to the present invention andcomparative examples: Table 6 shows the chemical compositions of steelsand Table 7 their properties. The examples according to the presentinvention and comparative examples were prepared by the same methods asin Example 1.

The evaluation of the rolling defects and the properties after thepost-rolling quenching and tempering was also done in the same manner asin Example 1.

After being drawn to a diameter of 4 mm, the steel wires were quenchedand tempered through a so-called oil quenching and tempering treatment,wherein they were quenched by being heated in a radiation furnace and,immediately thereafter, cooled in an oil bath and then tempered by beingheated in a molten Pb bath.

In the oil quenching and tempering treatment, the drawn wires were madeto pass through a heating furnace continuously, and the residence timein the heating furnace was controlled so that the wires were well heatedto the center portion. If the heating is insufficient, hardening becomesinsufficient, and a sufficient strength cannot be achieved. In thisexample, the heating temperature was set at 950° C., the heating time at150 sec. and the quenching temperature (oil bath temperature) at 50° C.Then, the strength of the wires was controlled through the tempering ata tempering temperature of 400 to 550° C. and a tempering time of 1 min.The heating temperature for the quenching and tempering and the tensilestrength in the normal atmosphere obtained through the treatment were asshown in Table 7. The tensile strength was controlled to 2,150 to 2,250MPa or so.

The globular carbides in the steels of the examples including cementite,which is considered to be of importance in the present invention, arealso shown in the table. The size and number of the carbides wereevaluated in the same manner as in Example 1.

The ductility of the samples was evaluated through the notch bendingtest shown in FIG. 4.

It is a common practice with high-strength springs to apply nitriding,after the formation into the shape of springs, for hardening the surfaceand enhancing durability. In this relation, steel wires having tensilestrengths controlled to 2,150 to 2,250 MPa were subjected to a nitridingtreatment for the purpose of examining their nitriding characteristics.Here, the so-called soft gas nitriding treatment was applied under thefollowing conditions: a nitriding temperature of 520° C., a holding timeof 3 h., a mixed atmosphere gas of N₂ 45%+NH₃ 50%+CO₂ 5%, and a gas flowrate of 1 m³/h. (at atmospheric pressure).

After the nitriding treatment, sectional planes of the steel wires werepolished in a mirror finish and the hardness was measured at theoutermost layer (25 μm from the outer surface) and an internal portion(0.5 mm from the outer surface) using a micro Vickers hardness tester(0.49 N). While the surface layer is hardened through nitriding, theinside tends to soften by the heating during nitriding. It is importantfor a spring steel that the surface layer is sufficiently hardened andthe softening of the internal portion is minimized.

In the invented examples, the number and size of globular carbides afterrolling were small, rolling defects were prevented from occurring and ahigh strength and a good notch bending property were realized after thequenching and tempering. It is seen from the table that comparativeexamples, in contrast, were inferior in the notch bending property andthe coiling property. Further, rolling defects were found in them,witnessing a difficulty in rolling the operation.

[Tables 6 and 7]

TABLE 6 Chemical composition (mass %) Example No. C Si Mn P S Cr W Ti VMo Nb Mg N Invented 104 0.89 1.71 0.77 0.010 0.005 0.79 0.10 — 0.0029example 105 0.85 1.89 0.83 0.009 0.006 0.65 0.19 — 0.0046 106 0.82 1.840.88 0.007 0.011 0.78 0.28 0.008 0.0010 0.0035 107 0.89 1.79 1.08 0.0120.013 0.80 0.22 0.014 0.0009 0.0030 108 0.81 1.76 1.10 0.005 0.007 0.950.19 0.0010 0.0044 109 0.83 1.82 0.88 0.014 0.004 0.92 0.30 0.15 0.00040.0041 110 0.87 1.68 0.74 0.009 0.011 0.75 0.28 0.02 0.0008 0.0042 1110.81 1.80 0.71 0.009 0.013 0.72 0.29 0.0009 0.0033 112 0.88 1.87 1.060.013 0.008 0.88 0.21 0.0008 0.0041 113 0.90 1.86 0.78 0.006 0.008 0.960.07 — 0.0038 114 0.86 1.84 0.71 0.005 0.012 0.80 0.16 0.007 — 0.0045115 0.82 2.03 0.32 0.004 0.013 0.68 0.14 0.07 — 0.0033 116 0.87 2.220.84 0.010 0.008 0.52 0.18 0.0009 0.0034 117 0.88 2.00 1.04 0.007 0.0130.82 0.18 0.0008 0.0045 118 0.78 1.46 0.84 0.011 0.004 0.79 0.12 0.00110.0031 119 0.84 1.50 0.71 0.009 0.005 0.95 0.29 0.25 0.40 — 0.0046 1200.87 2.35 0.93 0.015 0.011 0.68 0.09 0.15 — 0.0048 121 0.86 2.38 0.400.010 0.010 0.52 0.06 0.45 0.0008 0.0048 122 0.81 2.07 0.30 0.008 0.0130.84 0.29 0.30 0.24 0.0010 0.0028 123 0.85 2.22 0.71 0.006 0.004 0.620.25 0.10 0.17 0.0005 0.0044 124 1.06 1.68 0.35 0.014 0.010 0.55 0.180.0007 0.0037 Comparative 125 0.85 1.79 1.12 0.012 0.008 1.56 — 0.290.25 0.0032 example 126 0.75 1.54 0.76 0.008 0.007 0.80 — 0.38 0.35 0.060.0035 127 0.68 2.25 1.03 0.010 0.014 0.45 — 0.11 0.23 0.08 0.0036 1280.79 1.86 0.25 0.006 0.012 0.96 0.14 0.08 0.38 0.28 0.0033 129 0.70 2.101.07 0.012 0.004 1.81 0.12 0.41 0.12 0.0045 130 1.45 2.15 0.25 0.0120.009 1.00 0.16 0.40 0.36 0.0045 131 0.95 2.12 0.96 0.004 0.007 0.911.10 0.53 0.31 0.0042 132 0.82 2.01 0.99 0.010 0.008 0.15 0.11 0.120.0042

TABLE 7 Notch Nitriding property Density Density Tensile bending(hardness HV) (after rolling) Rolling (after OT) strength angle SurfaceInternal No. 0.2-3 μm >3 μm defects 0.2-3 μm >3 μm MPa (°) layer portionInvented 104 0.022 0.0014 ∘ 0.08 0.0014 2158 36 726 519 example 1050.060 0.0015 ∘ 0.16 0.0011 2262 34 765 539 106 0.005 0.0020 ∘ 0.020.0008 2226 35 728 526 107 0.043 0.0005 ∘ 0.13 0.0008 2209 36 759 495108 0.045 0.0007 ∘ 0.18 0.0016 2154 36 744 534 109 0.040 0.0012 ∘ 0.100.0014 2234 38 768 534 110 0.015 0.0005 ∘ 0.06 0.0016 2177 36 764 542111 0.057 0.0018 ∘ 0.15 0.0014 2151 31 730 493 112 0.018 0.0009 ∘ 0.060.0008 2153 31 770 508 113 0.053 0.0013 ∘ 0.19 0.0018 2175 37 748 523114 0.015 0.0013 ∘ 0.06 0.0007 2178 36 734 505 115 0.192 0.0017 ∘ 0.330.0014 2231 36 740 523 116 0.095 0.0008 ∘ 0.25 0.0011 2160 37 729 522117 0.027 0.0008 ∘ 0.08 0.0018 2207 40 731 521 118 0.071 0.0015 ∘ 0.250.0019 2219 38 764 504 119 0.156 0.0010 ∘ 0.28 0.0017 2214 35 727 496120 0.019 0.0012 ∘ 0.09 0.0010 2225 38 723 547 121 0.280 0.0012 ∘ 0.330.0017 2182 39 735 504 122 0.186 0.0012 ∘ 0.34 0.0012 2150 34 724 505123 0.246 0.0010 ∘ 0.77 0.0016 2241 39 744 507 124 0.019 0.0017 ∘ 0.060.0018 2163 40 742 530 Comparative 125 0.468 0.0057 x example 126 0.4050.0064 ∘ 1.21 0.0070 2167 23 746 528 127 0.594 0.0100 ∘ 1.77 0.0110 217928 720 537 128 0.005 0.0053 ∘ 0.01 0.0058 2209 28 728 513 129 0.0640.0131 x 130 0.546 0.0103 x 131 0.738 0.0103 ∘ 0.91 0.0113 2259 27 744493 132 0.006 0.0024 ∘ 0.05 0.0019 2214 37 547 433

As for the influence on nitriding, as seen in comparative example 132with a low content of Cr, the surface hardness after nitriding is lowerthan invented examples and, in addition, the internal portion was softerthan invented examples owing to insufficient temper softeningresistance. As the insufficient hardness in these portions isdisadvantageous compared with invented examples in terms of durabilityand settling resistance, unlike the case of invented examples, nitridingdoes not bring about an improvement in spring performance in suchsteels.

INDUSTRIAL APPLICABILITY

The present invention makes it possible to obtain a steel and a steelwire for springs having a high strength and a high toughness byadjusting a steel chemical composition so as to be able to control theprecipitation of carbides including cementite in the steel. The presentinvention also makes it possible to produce a spring having a highstrength after heat treatment. When applied, in particular, to thesprings manufactured by the cold coiling method, too, the presentinvention makes it possible to produce a high-strength spring excellentin fracture property by increasing the strength to 1,900 MPa or more andsecuring a good coiling property.

1. A spring steel wire having a high strength and toughness after heattreatment suitable for use in springs for cars, characterized by: thespring steel wire consisting essentially of, in mass, C: 0.4-1.2%, Si:0.9-3.0%, Mn: 1-2.0%, P≦0.015%, S: <0.015%, Cr 1.07-2.5%, N:0.001-0.015%, Mg: 0.0004-0.01%, W: 0.05-1.0% with a balance of Fe andunavoidable impurities; and wherein steel for spring steel wire has beensubjected to heating at 1200° C. or more for an effective amount of timeto dissolve coarse carbide in the steel followed by hot rolling; andwherein globular cementite carbides observed in a microscopic visualfield satisfy the density of globular cementite carbides of 0.2 to 3 μmin circle-equivalent diameter is 0.5 piece/μm² or less and the densityof globular cementite carbides of over 3 μm in circle-equivalentdiameter is 0.005 piece/μm² or less.